The present invention is a method of easily preparing aqueous solutions of anionic polyelectrolytes by dissolving alkali-soluble alkenoic acid-alkyl acrylate latexes in alkali metal hydroxide solutions.
These latexes are known from Fordyce et al. Official Digest 31:284-302 (1959), and Ser. No. 450,650 filed Mar. 13, 1974 now U.S. Pat. No. 3,926,891, dated Dec. 16, 1975. It is extremely difficult to prepare solutions from these latexes having a solids content higher than about 5% due to the extremely high viscosity obtained when alkali metal hydroxide is mixed into an alkali-soluble acrylic latex. In actual practice, the alkali-metal hydroxide coagulates the latex into a rubbery mass which then proceeds to slowly dissolve. Extremely powerful mixers are required to knead the rubbery polymer mass until it dissolves and the viscosity drops to a more manageable level.
The present invention restricts this temporary rubbery phase to only a fraction of the material being dissolved at any given instant. This is accomplished by mixing small increments of the two components (latex and alkali metal hydroxide) on a continuous basis thereby avoiding the need for expensive and large equipment. Production rates in excess of 100 gallons per hour can readily be achieved in equipment using only small diameter piping.
In view of the usual higher raw materials cost for alkenoic acids compared to alkyl acrylates, it is advantageous to use less alkenoic acid in the latex preparation than is desired in the final polyelectrolyte solution. The process of the present invention readily accomodates this change by allowing for the heating of the newly formed polyelectrolyte solution until the pH stabilizes. During this digestion stage the alkali-metal hydroxide, present in excess of the alkenoic acid, reacts with the alkyl acrylate in a saponification reaction thereby producing more alkenoic acid salt in the copolymer.